Emission regulations have become more and more strict every year over the world in light of environmental problems. Inparticular, it is imperative to reduce NOx and particulate matters (PM) in the exhaust gas from diesel engines. For reducing such exhaust gas, combustion improvement technologies, such as high-pressure injection and exhaust gas recirculation (EGR) systems, or exhaust gas treatment technologies, such as oxidation catalysts, diesel particulate filters (DPF), and NOx storage reduction catalysts, have been under development.
DPFs, which are used for capturing and removing PM in exhaust gas with a filter, have various structures. For example, a wall-flow type DPF has a structure wherein the two ends of a honeycomb monolith are alternately plugged. However, this type of a DPF has problems in that the captured PM accumulates to increase pressure loss, lower engine power, or deteriorate fuel efficiency. For solving such problems, a DPF per se is replaced after a certain mileage, or detached and regenerated to remove PM by oxidization and combustion. Recently, continuous regenerative DPFs have been developed that are intended for mounting on vehicles and continuously regenerated while driving.
Motor vehicle diesel fuels have been in compliance with regulations with regard to SOx, NOx, and PM, and their sulfur content has gradually been reduced from 0.2 mass % to 0.05 mass %. Recently, diesel oils with 0.005 mass % sulfur have become popular.
Lubricants for diesel engines contain lubricant base oil and various additives blended therein, which include various active components, such as metals, sulfur, and phosphorus. For example, JP-6-49476-A, JP-7-10273-A, JP-8-48989-A, JP-2000-119680-A, JP-2000-256690-A, and JP-3-62893-A propose low-ash lubricants for diesel engines with lowered metal contents for reducing PM accumulation on a DPF.
Low-ash lubricants for diesel engines proposed so far are effective in reducing ash settlement on a DPF. However, it has been revealed that deposition of ash on a DPF cannot be inhibited merely by lowering the ash content. Such deposited ash cannot be removed easily by back flushing operation by means of air blow in the regeneration. The deposited ash has severe impact on the lifespan of a DPF, particularly of a continuous regenerative DPF. Thus control of formation of the depositing ash is essential. However, details of the depositing ash formation has not been fully investigated so far, and little report has been made to date on actual evaluation and optimization of the impact of fuel and lubricant on diesel engines equipped with a regenerative DPF.